Testing system



Nov. 26, 1935. ALMQUIST ET 2,021,978

TESTING SYSTEM Filed Dec. 1, 1932 20 EZz'J/Le INVENTORS- ATTORNEYPatented Nov. 26, 1935 UNITED STATES PATENT OFFICE Clifton, N. J.,assignors to AmericanTelephone and Telegraph Company, a corporation ofNew York Application December 1, 1932, Serial No. 645,296

5 Claims.

This invention relates to electrical testing sys tems. Moreparticularly, this invention provides means for lay-passing directcurrents or very low frequency alternating currents at telephonerepeater points in order topermit tests to be made on the linesextending to those points.

In accordance with this invention arrangements are provided wherebytests can be made on lines having for example, an unattended repeaterstation placed between two testing points. This invention isparticularly adaptable in the case of cable carrier systems whereadditional stations for carrier repeaters are required between theattended stations now provided on existing cable routes. These addedstations are unattended except for periodic visits of a maintenance man.In such cases it is not possible to make fault location and othersimilar tests common to this type of plant without providing meanssomewhat similar to that described hereinafter, or without sending atest man to the unattended ofiice. In many cases of an emergencycharacter the latter procedure would add considerable delay and expenseas well as contribute to the possibility of complete failure of thefacilities.

The nature of the tests which it is contemplated can be made by means ofthis invention is primarily concerned with the detection and location offaults, but normal routine tests of insulation, series resistance, etc.,can be made as well. In addition, channels can be so provided for conetrol of unattended equipment, for the transmission of power, or, inshort, for any operation requiring a direct current or a low frequencyalternating current path. It is to be pointed out that these functionscan be performed without in any way interfering with the normaloperation of the repeater, if such is the desired result.

It becomes necessary in such a system to keep the by-pass circuit low inresistance and at the same time introducing sufficient high frequencyalternating current loss so that the singing margin in the transmittedband is great enough not to change the gain-frequency characteristic ofthe amplifier and the margin outside the band sufiicient to preventsinging. In the embodiment of the invention disclosed in thisapplication, this purpose is accomplished by means of a networkconsisting of series resistances and. parallel capacitances in asufficient number of elements to afford the desired attenuation. The useof other equipment, such as filters having a greater or smaller numberof sections employing'inductance coils or inductively wound resistances,and in which any or all of the condensers may be of the well knowncompound type in which the midpomt or midpoints are grounded in order tore-' duce longitudinal induction, or of other filter arrangements wellknown in the art, shall not be.

number of available embodiments merely for the purpose of illustration.

The embodiment to be used for the purpose of illustration is shown inFigures 1 and 2 of the accompanying drawing. Figure 1 describes anarrangement for a station around which it is desired to make tests andFig. 2 describes anarrangement whereby the through arrangement of Fig. 1can be split for the introduction of testing or other equipment. Thus,it is possible to use the same kind of equipment at testing andnontesting stations or readily to arrange to convert a non-testingstation to a testing station, without affecting the normal transmissionover the circuit, if so desired. In this connection it may be pointedout that it is contemplated that the order of frequencies to betransmitted over the normal circuit may range from voice frequencies toany high frequencies as, for instance, frequencies up to 100 kilocycles.

With reference to both Figs. 1 and 2, the reference character Idesignates an amplifier of any known type and 2 an equalizer which,while shown as one of the elements in the drawing need not be includedin the circuit, or if it is included, it may, if desired, be connectedin bridge arrangement with the amplifier rather than as shown. In thisapplication a one-way amplifier is preferred, but the invention does notpreclude the use of two-way amplifiers with, for example, theirassociated hybrid coils. In any event, it will, in general, be necessaryto couple the amplifier to the lines by means of coils, and these areshown as input coils at 3 and as output coils at 4, in the line sides ofwhich are included condensers 5 and 6.

The line connections of Fig. 1 are then effectively connected togetherfor direct current and very low frequency alternating current purposesby resistances 1 and 9 and 8 and I 0. A third condenser ll 'isintroduced across the line as shown. It follows that the high-frequencyalternating current attenuation between the lines, i. e.,' theattenuation from the input side to the output side of the repeater, canbe made very great,

of the order of 100 decibels or more, and of comparatively no loss fordirect current. Thus, the amplifier will not sing due to the nature ofthe connection from its input circuit to its output circuit, nor is thehigh-frequency transmission characteristic appreciably different withthe bypass connected around the amplifier or entirely disconnected fromit. Yet when the by-pass circuit is included it is possible to performtests around the amplifier, using direct currents or low-frequencyalternating currents.

In the arrangement of Fig. 2, the condenser ll of Fig. 1 is replaced bytwo condensers I2 and [3, so that the two lines extending to theamplifier can be segregated in order that either may be testedindependently, if so desired.

A characteristic set of values for the elements of the by-pass circuitswhen associated with an amplifier which may amplify currents offrequencies between 4 and 100 kilocycles may include 100 ohms each forthe resistances I, 8, 9 and ill and 1 microfarad of capacitance forcondensers 5, 6 and i 5. These values are given merely for illustrationand do not represent limitations '1 on the scope of the invention, forother values may be assigned to these elements, if desired.

It is, of course, important that the resistances i and 9 be carefullybalanced against resistances 8 and Hi and their absolute values wellknown and constant in character so that their magnitudes may beaccounted for and adequate. allowances made in fault location work.Condensers 5, 6 and II and the condensers i2 and 23 should be possessedof high insulation resistance and the insulation resistances of coils 3and 4 should be such that the insulation resistances of one of theconductors of the line to the other conductor of that line and to groundare not appreciably affected. If alternating current measurements are tobe made employing a low frequency, such as, for instance, a frequency offour cycles per second, the capacitance of the condensers 5, and H, orthe capacitance of condensers l2 and I3, should be known andsubstantially constant in their absolute value of capacitance.

tion consist in eliminating from the gross measured resistances theknown values of resistance r of the introduced networks, leaving theresistive value of the line alone which is then used in any one of anumber of well known ways in order to determine the location of thefault. Similar means can be employed in the case of low-frequencyalternating current measurements. It is assumed in the case ofinsulation resistance measurements that the insulation resistances ofthe equipment can be neglected.

While this invention has been described in certain arrangements merelyfor the purpose of illustration, it will be understood that the generalprinciples of this invention may be applied to other and widely variedorganizations without departing from the spirit of the invention and thescope of the appended claims.

What is claimed is:

1. The combination of an amplifier through which are transmittedcurrents of one frequency band, two lines extending from differentdirections to said amplifier and coupled to opposite sides of saidamplifier, two auxiliary lines each connected to one of the lines andcoupled to said amplifier, each of said auxiliary lines including,

series resistance elements and shunt capacitance elements for freelytransmitting currents within a second frequency band and preventing thetransmission therethrough of currents within the first frequency band,and testing equipment which 5 may be connected to either or both of saidauxiliary lines.

2. A system for testing a circuit which includes an unattended repeaterand not permitting any of the testing currents to pass through therepeater, comprising a by-p-ass circuit bridged across the repeater andin series with the circuit to be tested and adapted to freely transmitdirect currents, said by-pass circuit including two parallel conductors,two equal non-inductive resistances one of which is in series with eachof the conductors, and a condenser bridged across said conductors.

3. In an arrangement for testing a circuit which includes an unattendedrepeater station at which there is an amplifier for amplifying speechand high frequency currents, the combination of a first circuitextending to said amplifier, a second circuit extending from saidamplifier, third and fourth circuits each formed of two conductors whichare shunted across said first and second circuits, respectively, andmeans comprising equal resistances in each of said conductors includedin said third and fourth circuits and a condenser directly bridgedacross the junctions of saidequal resistances for permitting the freetransmission of direct currents and alternating currents of frequencieslower than the frequencies amplified by the amplifier, the directcurrents and alternating currents of lower frequencies being employedfor testing purposes.

l. The combination of an amplifier which cmploys speech currents orcurrents of high frequencies, a line extending to the amplifier from onedirection, another line extending to the amplifier 40 respectively. i,

5.- The combination of an amplifier for high frequency alternatingcurrents, two lines extending in different directionsfrom the amplifierandcoupled to opposite sides of said amplifier,

means for independently transmitting direct currents or low frequencyalternating currents over said lines for testing said lines withoutinterfering with the amplification of the high frequency alternatingcurrents by said amplifier, said means including two two-wire circuits,respecti (so bridged across the two lines extending to said amplifier,each two-wire circuit including two equal non-inductive resistances anda condenser connected in shunt with said resistances for preventing thetransmission therethrough of the high frequency alternating currentsamplified by the amplifier and for freely transmitting the directcurrents or low frequency alternating currents employed for testingpurposes.

MILTON L. ALMQUIST. PAUL G. EDWARDS. CHARLES E. CLU'I'IS.

